In the base station of portable telephones, etc., using the quasi-microwave band or the microwave band, a directional coupler is used in order to monitor the transmission power at the base station. A high-frequency front end portion in the base station of portable telephones, etc., is composed of a transmission or reception filter using a dielectric resonator, a low-noise amplifier, etc., and connected to a transmission and reception antenna. The high-frequency front end portion monitors whether the base station transmits an electric power necessary for making the communication in a fixed area possible and the circuit is constructed so as to be able to stably transmit an electric power on the basis of the monitoring result. The directional coupler is used for monitoring the electric power transmission and disposed between the transmission and reception antenna and the high-frequency front end portion. Furthermore, as a coupling line for making the directional coupler coupled to a main line inside the circuit, a microstrip line characterized in that the production is easy and that the coupling to lines in various shapes can be easily obtained is often used.
In Patent Document 1, in a circuit using a waveguide as a main line, a directional coupler in which a microstrip line is inserted into the waveguide is shown. When a microstrip-line type coupling line is inserted into a waveguide, the electromagnetic field inside the waveguide is coupled to the microstrip line at high frequencies and a part of the electric power inside the waveguide can be taken out.
However, when a microstrip line is inserted in a waveguide, there was a problem in that it becomes difficult to specify the directivity to the waveguide because of the influence of the grounding electrode on the back surface of the substrate. Then, in patent Document 1, the directivity is improved in such a way that the whole grounding electrode on the back surface in the length direction of a coupling line portion where the electromagnetic field of the waveguide is coupled to the microstrip line is made to retreat a fixed distance in the width direction of the coupling line portion. When a waveguide and microstrip line of fixed dimensions are used, it is understood that the directivity is improved up to 20 dB by making the grounding electrode on the back surface retreat a fixed distance in the width direction of the coupling line portion. Furthermore, in Patent Document 1, although the grounding electrode on the back surface is made to have a fixed shape for the purpose of improving the directivity at connection to the waveguide, also the same effect can be obtained in the structure where, instead of the waveguide, the center conductor of a coaxial line is made a main conductor.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2-26103
However, in the structure of Patent Document 1, since the grounding electrode on the back surface is made to retreat in the line width direction over the whole coupling line portion where the microstrip line and the waveguide are coupled, there is a problem in that the directivity greatly changes by a little positional displacement between the microstrip line and the grounding electrode when the electrode pattern is formed. The problem is described by using the structure in FIGS. 5A and 5B where the structure in Patent Document 1 is used for coupling to the coaxial line.
FIGS. 5A and 5B are schematic sectional views of a substrate surface where the line portion of a microstrip line coupled to a coaxial line is formed is cut as a sectional surface. In a directional coupler having the structure as in FIG. 5, in order to obtain the directivity of a current flowing in a microstrip line 40, the strength of a magnetic field coupling and the strength of an electric field coupling generated between the center conductor 42 (hereinafter, referred to as the main line) of a coaxial line 41 and the microstrip line 40 are required to be made equal to each other. FIG. 5A shows the direction of a current flowing in the microstrip line 40 when both lines are coupled by a magnetic field generated in the main line 42. A circular magnetic field 44 is generated around the main line 42 by a current flowing in the main line 42. A substrate 45 having the microstrip line 40 formed is inserted in the magnetic field and, when the microstrip line 40 is brought close to the main line 42, the main line 42 and the microstrip line 40 are coupled by the magnetic field 44. At this time, an induced current 46 is generated in a coupling line portion 47 of the microstrip line 40. The induced current 46 flows from one end of the microstrip line 40 to the other end.
On the other hand, FIG. 5B shows the direction of a current flowing in the microstrip line when the main line and the microstrip line are coupled by a capacitance generated between both lines. When the microstrip line 40 is brought close to the main line 42, a coupling capacitance 48 is generated between the main line 42 and the microstrip line 40 and an electric field coupling is caused between the lines. At this time, since a substantially symmetrical electric field strength distribution is obtained over from the middle point of the coupling line portion 47 to both ends 49 and 50 of the microstrip line 40, the currents 51 and 52 of the same magnitude are generated in the same direction at both ends 49 and 50 of the microstrip line 40.
When a directional coupler is constituted by close arrangement of a main line and a microstrip line, both a magnetic field coupling and an electric field coupling occurs, and currents corresponding to those flows in the microstrip line. In FIGS. 5A and 5B, when the amount of electric field coupling and the amount of magnetic field coupling are the same, since the amount of a current 46 flowing into the other end 50 of the microstrip line which is generated by the magnetic field coupling and a current 51 flowing into one end 49 of the microstrip line which is generated by the electric field coupling become substantially the same, the current to one end 49 does not flow and the current only to the other end 50 flows. Therefore, the directivity of a current flowing in the microstrip line is decided and the directivity of the directional coupler can be obtained. Then, when a monitor circuit is connected to the other end 50, it is able to monitor the electric power 43 passing through the main line 42.
In Patent Document 1, the electric field strength between the microstrip line and the grounding electrode is changed by making the grounding electrode opposite to the coupling line portion retreat a fixed amount in the line-width direction, and thus, the magnetic field coupling amount and the electric field coupling amount between the microstrip line and the main line are made equal to obtain the directivity. However, since the whole grounding electrode opposite to the coupling line portion is made to retreat, the amount of change of the magnetic field coupling amount and the electric field coupling amount generated between both lines which is caused by the amount of retreat of the grounding electrode becomes large. Therefore, when positional displacement between the grounding electrode and the microstrip line occurs in formation of the electrode pattern, etc., since either of the magnetic field coupling amount and the electric field coupling amount between both lines becomes larger, there occurs a problem in that the directivity cannot be obtained.